[Nescafe]

Hi,

I always get confused whether these two are interchangeable or not.

EC50 is the concentration of inhibitor required to reduce cellular effect resulting from enzyme activity by 50%.
IC50  is the concentration of inhibitor required to reduce the activity of the enzyme by 50%.

In enzyme studies IC50 is the concentration of inhibitor where we see enzyme activity is reduced by 50%. EC50 is the concentration of inhibitor at which we see a 50% decline in the cellular activity resulting from enzyme activity. So if I am understanding this, inhibiting enzyme activity by 50% does not necessarily mean that the cellular activity will also be reduced by 50%? So in this case EC50 and IC50 are not the same?

I am confused ;(

Nescafe.

[jhcoleman53]

Hello,

I’d say it all depends on what terminology you prefer, and what is required by the study, any applicable regulatory requirements, etc.  For example, if you define the “effect” as a 50% reduction in cellular activity, then you can use EC50.  If you use the terminology such as a 50% “inhibition” in cellular activity, then IC50 would be more appropriate.  Likewise for enzyme activity.  I don’t think you should mix the two within the same project, though.  You should pick one or the other, and be specific when you use them, e.g.,  “EC50 (cellular activity)” and “EC50 (enzyme activity)”; or IC50 if you wish.  And you could very well be right that a 50% reduction in enzyme activity does not correlate directly to a 50% reduction in cellular activity, but that’s a different subject than IC50-or-EC50.

[fledarmus]

EC50 is 50% efficacy, and IC50 is 50% inhibition. The efficacy dose is determined by measuring the effect in a living system, the inhibition dose is measured on isolated enzymes.

You are right - inhibiting the enzyme by 50% does not automatically lead to 50% efficacy. For some enzymes, you have to get greater than 90% inhibition to see any effect at all in the cell or animal.

[jhcoleman53]

I think the meaning of the two terms is something that has to be taken totally within the context of what you are studying.  For example, the previous poster stated that EC50 refers to 50% efficacy, and this is probably true for whatever type of investigation is being referred to.  However, in some toxicology studies, an EC50 is known as an 50% EFFECT concentration .  Different strokes..

[Nescafe]

very helpful, thank you guys!